In mobile communication, a coding gain is often improved to increase a reception performance on a poor transmission path such that a demodulation circuit is devised and error correction is performed based on a soft decision value. A code outputted as a soft decision value is sent to an error corrector as data which indicates an accuracy of 0 to 7 and in which 1-bit data outputted as 0 or 1 in a hard decision is extended to 3 bits for example. The accuracy can be obtained by quantizing a demodulated continuous wave by an appropriately determined threshold value.
FIG. 7 illustrates a relationship between a probability distribution and a soft decision value as to a binary in the case of a binary code. The abscissa in the graph of the figure denotes an amplitude of a demodulated wave and the ordinate denotes probability. Reference numeral 71 in the figure denotes a curve expressing a probability distribution in the case where a demodulated wave is 1 and reference numeral 72 denotes a curve expressing a probability distribution in the case where the demodulated wave is 0. Numerals 0 to 7 are soft decision values allotted to the amplitude of each amplitude range. In general, the soft decision for a binary code by BPSK or the like is performed by outputting the soft decision value allotted to the amplitude of the demodulated wave as illustrated in FIG. 7. A soft decision value 0 for a code 0 is most probable and a soft decision value 7 for the code 0 is most improbable. A soft decision value 7 for a code 1 is most probable and a soft decision value 0 for the code 1 is most improbable.
FIG. 8 illustrates an example of soft decision for the case of a four-level code. For multilevel coding such as four-level coding, for example, an operation similar to one in FIG. 7 is performed on a complex plane. That is to say, as illustrated in FIG. 8, the soft decision, similar to that in FIG. 7, of a received signal is performed both at a real part and at an imaginary part and the soft decision values may be outputted at the respective parts. Such a technique is used in Patent Document 1 for example. Furthermore, such a technique is discussed in a conventional example of Patent Document 2.
FIG. 9 illustrates an amplitude in the case where a demodulated wave is viewed by a time waveform and the correspondence of the amplitude to the soft decision value allotted to the amplitude. Thus, the four-level is allotted to an amplitude value of a linear time waveform to enable the soft decision value to be obtained based on the amplitude value of the time waveform. This is applied to the case in FIG. 8, which is equivalent to the arrangement of the threshold value of each soft decision value at an equal angle.
Incidentally, herein, a rate in which the amplitude of a demodulated wave is allotted to each soft decision value is referred to as “weighting.” The allotment of each soft decision value to each range in which the amplitude of a demodulated wave is divided by an equal threshold value, as in the aforementioned example, is referred to as “linear weighting.” FIGS. 8 and 9 are different in the allotment of weighting from each other. In each case, a certain criterion is equally allotted, so that each case is treated as linear weighting.
Error correction in a soft decision decoding method has been often used in a Viterbi decoder, for example. The Viterbi decoder processes a soft decision value as a metric, adds the metric for each bit and completes a trellis. Since the metric is a distance between codes, it is desirable that the metric is the probability for the code word.
However, according to a conventional linear weighting concerning a soft decision value, a threshold value is arranged at equally spaced intervals as illustrated in FIGS. 7 and 8, so that the soft decision value does not fully represent an actual probability. For example, the graph of FIG. 7 illustrates a probability distribution in which a demodulation bit is 0 or 1 with respect to the amplitude of a demodulated wave on the abscissa under the condition of a certain error rate. If this is rewritten to the probability that a demodulation bit is 1, FIG. 10 is obtained. In other words, if the soft decision value is 0 to 2, the decoded word is substantially 1. If the soft decision value is 5 to 7, the decoded word is substantially not 1. The probability is reversed between the soft decision values of 3 and 4, which shows that the soft decision value cannot represent the probability.
In order to solve the above problem, the soft decision unit in Patent Document 2 is equipped with an S/N detecting unit in its receiver to vary the threshold value of the soft decision according to reception environment, avoiding the above problem. According to the above method, however, a circuit such as the S/N detecting unit needs adding. Furthermore, a plurality of threshold values for soft decision need preparing to perform a complicated process.
FIG. 11 illustrates the measured values of the probability that the demodulation bit of four-level FSK is 1 at a certain error rate with the abscissa as amplitude and the ordinate as probability. Bit arrangement corresponding to the amplitude is described later. According to the conventional soft decision method, for the four-level FSK, the soft decision value is not changed if the amplitude is −3 or less and 3 or more. Actually, however, the probability is decreased due to the intensity of Gauss noise or multipath fading. According to the conventional soft decision method, it is also difficult to reproduce this portion.
According to a decoding apparatus and decoding method in Patent Document 3, the dispersion of actual quantization noise and Gauss noise is determined by calculation to perform the soft decision decode for the purpose of improving the accuracy of turbo decoding. Thus determining the dispersion of Gauss noise allows calculating the probability of a code word from a demodulated wave and also accurately representing decrease in the probability if the amplitude value is large, which however causes a problem that significantly complicates the calculation.    Patent Document 1: Japanese Patent Application Laid-Open No. H10-136046    Patent Document 2: Japanese Patent Application Laid-Open No. H06-29951    Patent Document 3: Japanese Patent Application Laid-Open No. 2005-286624